1. Field of the Invention
The present invention relates to intake control devices and methods for changing a length of an engine air intake conduit to vary an output characteristic of the engine. The present invention also relates to a straddle type vehicle employing such devices and methods.
2. Description of the Related Art
Certain straddle type vehicles such as, for example, motorcycles, employ an engine air intake control device in which a length of an intake conduit is changed from a first length to a length shorter than the first length when an engine load (such as, for example, an engine speed or a throttle valve opening) reaches a predetermined threshold value. Thereby, an output characteristic of the engine can be varied. One example of such a variable length intake system is shown in Japanese Publication No. 09-100720.
FIG. 8 is a graph illustrating an example of a control process for changing a length of an intake conduit according to a prior intake device, such as the device of Japanese Publication No. 09-100720. The horizontal axis of the graph indicates throttle valve opening and the vertical axis of the graph indicates engine speed. The intake control device changes a length of an intake conduit from a first length to a length shorter than the first length when, for example, the throttle valve opening exceeds a predetermined threshold value Th and an engine speed exceeds a predetermined threshold value Ne (i.e., when a running state reaches an area indicated by the letter A in FIG. 8). Afterwards, the intake control device returns the length of the intake conduit to the first length when the throttle valve opening falls below the threshold value Th or the engine speed falls below the threshold value Ne (i.e., when the running state reaches an area indicated by the letter B in FIG. 8).